1. Field of the Invention
The present invention has application in those process industries wherein gas streams are encountered which contain gaseous contaminants and exhibit a temperature in the range from about 400.degree. F. to 3000.degree. F. A specific example of such a process gas stream would be that discharged from a waste product incinerator which contains both products of combustion and also gaseous contaminants as a result of incineration of the aforesaid waste product. As is well-known to those working in this general area, both local and federal government regulations regulate strictly those materials which may be discharged into the atmosphere and, accordingly, provision must be made to treat such process gas streams in order to bring their composition into compliance with applicable regulations. Moreover, in many instances, the process gas streams will contain valuable constituents which may be advantageously recovered for reuse or sale.
A particular problem which exists in treating process gas streams of the type under consideration is that, during operation of the waste product incinerator or other apparatus involved, the discharge therefrom of the process gas stream may be subject to substantial variations in flow rate. In order to accommodate this problem, it is desirable that the treatment method and apparatus be sufficiently flexible as to handle adequately the aforesaid substantial variations in process gas stream flow rate.
A second problem in treating process gas streams of the type under consideration is presented if recovery of the particular contaminants involved is desired. In this event, it is necessary that the absorbent liquid reach at least a minimum level of concentration in order to facilitate such recovery. Obviously, this requirement may dictate the desired, proper flow rate of absorbent liquid through the treatment apparatus; however, such flow rate must also be adequate to provide adequate absorption efficiency in the apparatus and, in some installations, may also have an effect upon the proper cooling of the process gas stream, as will appear hereinafter.
2. Description of the Prior Art
At the present time, an acidproof brick lined packed tower, similar to the first treatment column of this invention, has been used almost exclusively for process gas cooling and contaminant removal. The drawback of this method is that the unit can be operable only over a range of 50% to 100% of the designed process gas flow rate without losing scrubbing efficiency. A second drawback of this method is that the packings require a minimum wetting rate. If the fresh make-up liquid rate is lower than the minimum wetting rate of packing, it has to be satisfied either by increasing the fresh liquid rate, which will decrease the concentration of the liquid recovered; or by recirculating the quench/absorbent liquid, which will reduce the efficiency of the absorption column. Systems embodying two packed bed towers, either in parallel or in series, have also been used to solve the problems described by selectively deactivating one of the towers at low flow rates. Also, packings of variable sizes in one tower have been used but this approach risks the flooding of the tower.
To increase the operable range of the process gas flow rate variations, fresh air has been used to boost up the mass flow rate of the process gas. However, the required air flow rates are usually three to four times higher than the maximum exhaust gas flow rates. Thus, high operating and capital costs limit the use of this method.